The present invention relates to a method of feeding asynchronous motors with an inverter, in particular for battery-powered vehicles.
The technique of driving the three-phase bridge forming the inverter is independent of the type of electronic power component used and consists in the generation of a sinusoidal waveform or other type of waveform by means of the known PWM (Pulse Width Modulation) technique.
In the known feeding methods, the three-phase bridge introduces distortions into the voltage at the terminals which are reflected in a distortion of the current.
These distortions are due to the nature of operation of the inverter in which the two switches (electronic switch) belonging to the same bridge means must be driven in a complementary manner so as to prevent them from conducting together with consequent possible damage of the inverter.
The causes of the abovementioned distortions are to be found in the switching-on and switching-off times of the switches which are not ideal, with a consequent difference between the instant of actual commutation of the connection terminal of the motor and instant of commutation of the driving PWM. In addition to this, the commutation times vary depending on the load, thus resulting in a sinusoid which is not only different from that which is desired, but is also distorted.
A further cause of distortion arises from the need to introduce a dead time between switching-off of one switch and switching-on of the other one since, on account of the already mentioned commutation times, the two switches could in fact conduct together.
Although the dead time introduced in the PWM is known, the effective dead time cannot be defined with precision and is variable with the load conditions of the bridge on account of the presence of the delays in switching on and switching off. During the dead time periods, conduction of the current occurs via the recirculating diodes, so that the voltage at the terminal, during these periods, is defined by the sign of the current. The result is that the dead time adds to or subtracts from the voltage on the terminal a value depending on the sign of the current. With each change in the sign of the current, namely at each halfwave of the sinusoid, a step is generated in the voltage applied to the phases.
The abovementioned distortion affects operation of the motor, in particular at low voltages, or at low frequency, with a consequent distortion of the current and increase in the losses in the motor. Moreover, a distortion is produced in the rotating field and hence in the torque, which causes vibrations of the motor and the machine both at low speed and during the start-up or stopping phases.
Methods which are designed to reduce or eliminate the distortion effects described above are known.
One of the most widespread methods involves feedback of the filtered phase voltage and adjustment of the PWM with reconstruction of the sinusoid in the phases. On the basis of the feedback of the variables at the last stage in the chain, excellent results may be obtained, but the system is very complicated, and hence costly, in terms of both hardware and software. In particular, the electronic transduction device (filters and amplifiers), in addition to being complicated, causes delays and lack of homogeneity between the various parameters fed back due to the tolerances of the components; the adjustment of the control parameters is complex and it is difficult to find the right balance between speed of compensation and stability of the system. Also, in the case where a microcontroller is used, the control algorithm is complex and laborious and the microcontroller itself tends to be overloaded and it is often necessary to adopt a more powerful microcontroller with an consequent increase in costs.
Other known compensation techniques are able to read the sign of the current and, with an change in the sign, subtract from or add to the predefined amplitude of the sinusoid a value corresponding to the dead time.
This technique is very economical, but the results are not as good because, on account of the distortions, it is difficult to know the exact instant of inversion of the sign and also because the compensation value to be introduced is not defined.